I’m a big rocket nerd and have watched a lot of launches and every time I see one, the flames produced by the engines seem to be just as large as the size of the rocket. Is this simply a coincidence or is there a reason behind it?
In: Engineering
I don’t think they are. Obviously, bigger rockets need more thrust and more powerful engines, so the flame is bigger, but it’s not specifically as large as the rocket.
The size varies a lot depending on fuel type as well as where the rocket is. (At higher altitudes the flame spreads out, looking almost umbrella-like, and on the moon it was basically invisible)
If you compare (most) American rockets to Russian ones, the former generate what look like huge explosions, while the latter produce something more like a thin, precise stream behind them, because they use different types of fuel.
If you look at videos of the Shuttle launches, the flame was probably 3-4 times longer than the Shuttle or its booster. Looking at the SpaceX Starship test launches, the flame looks significantly shorter than the length of the rocket.
There’s a lot of variance, but I think the bottom line is that if you’re hurling tons of superheated gases out every second at high speeds, they’ll be able to produce a visible flame behind you for about 50-60 meters or so, before either spreading out or cooling down too much to be visible, which is in the same ballpark as the height of most rockets.
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